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My EL84 Triode PP Amp

sorenj07

Member
2006-05-04 9:15 pm
Berlin
craigslist deleted the ad, possibly for it being too long and detailed :) here it is for now... (asking $475)


A completely custom, hand-built and point-to-point wired tube amplifier. The chassis was lovingly pieced together using what was once a Tektronix 2213 oscilloscope. I really enjoy it in my current setup, but need to finance other audio projects I'm working on so I have to let it go...

[IMGDEAD]http://img515.imageshack.us/img515/2553/el84tmppfrontangleff2.th.jpg[/IMGDEAD] [IMGDEAD]http://img504.imageshack.us/img504/5493/el84tmppbackanglebg6.th.jpg[/IMGDEAD] [IMGDEAD]http://img87.imageshack.us/img87/5817/el84tmppwiringuz5.th.jpg[/IMGDEAD]

Schematic:
[IMGDEAD]http://img175.imageshack.us/img175/952/el84triodeppsignalmarkitf3.th.jpg[/IMGDEAD]

This amplifier uses triode-wired EL84 tubes, for a bit over 6W (4.8W Class A). Distortion of the output stage with no negative feedback connection simulates at less than .5% (3rd) and .05% (5th). The tubes are run in fixed-bias, with individual controls precise to the .1mA level. The driver stage uses a 6DJ8/6922 in a long-tail pair phase splitter, with a very high negative voltage supply which decreases phase imbalance. The output transformers were designed for EL84's in pentode mode at around 15W, and derating by 60% can only further increase bandwidth.

I use high quality PRP resistors in one of the more critical spots, namely the plate load resistors for the phase splitter. Wiring is largely done using silver-plated teflon-jacket high purity copper interconnect. The coupling capacitors are the well-known Russian K42-Y glass-sealed paper-in-oil types. The power supply is very robust - it uses a pure choke-input topology, using a monstrous Hammond 193M, rated for 300mA (174% of the whole amp's quiescent current draw). The power transformer is rated for 175mA in a capacitor-input supply but should be good for well over 200mA in choke input. Since the amp operates largely in Class A, strain on the PSU is further reduced.

Inputs are two RCA's. Volume is adjusted with a high quality Noble potentiometer. A CD player connected directly to the amplifier is more than capable of driving it to very loud listening levels, but of course this also depends on speaker efficiency. Separate filament and high-voltage switches help prolong tube life.

So how does it sound? First of all, the amp is DEAD QUIET on 98dB-efficient speakers. The power transformer makes a tiny humming noise, but all the transformers are isolated from the chassis with rubber grommets so mechanical induction is not a likely problem. It has plenty of bass for only 6W or so, but where it really shines is in the vocals. Voices really leap out of the speakers at you. Granted, this isn't the singer's-breath-on-your-cheek experience I've had with McIntosh amps feeding insane B&W speakers, but for a fraction of the expense, who's complaining? Highs are very smooth but clear - just listen to some hi-hat/cymbal-heavy jazz and you'll see what I mean. I'm no expert on 3D soundstages, but this amp definitely has something that I haven't heard very often. I think you'll like it.

Local pickup and cash transactions are MUCH preferred. Feel free to set up a time to give it a listen! I have a pretty wide variety of music (a fair bit of MFSL/audiophile quality recordings), but your own reference discs are fine also. I'm using a modified Playstation 1 as source, and Cerwin-Vega D-7's as my monitors. <br><br>I can also email the complete PDF for the output stage simulation. A final note: since I designed and built this amplifier, you're free to come by and ask me to take a look at it at any time should you choose to purchase it. Of course, if the amp malfunctions or breaks due to a defect on my part, I'll repair it.

Thanks for looking!